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Odd issue with Briggs 8FB engine

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  • Odd issue with Briggs 8FB engine

    Briggs 8FB, apparently low hours. Got it fairly recently for a low price. If will run, but not well.

    Piston is very draggy in the cylinder. Moves more reasonably freely when moving, but if allowed to stop, has a peculiar stick-slip feeling that is unlike any other engine I have worked on. Does not like to idle, but runs OK otherwise. Engine is cast iron.

    I suspect someone has been in the engine before, but I have no idea for what nor what they may have done.

    So, No part seems different from other engines. Cylinder is clean, no gouges, original 2.250" size in unworn areas, not bored out to an oversize, has a few thou wear, mostly in the middle as one would expect, slightly egged. Piston looks good, rings are not carboned up, they are free, but ring slots are within spec, rings were installed correctly. Ring gaps are toward the replacement size, but still in spec. Rings not scuffed looking and appear to be correct.

    Rod is good, not worn. Slight scuff in middle, but not too tight, oiling seems fine. Crank shaft rotates freely when piston is out.

    Piston was hard to remove. Top and second ring do not ehow the stick-slip, they have the usual feel if they are the only rings in place. Oil ring alone, with no other rings, gives the stick-slip when moving the piston by hand. Ring and spring look good, are not in any way odd looking. I dosed it with lots of 30 ND down the plug hole when I got it, there is no lack of oil, dipper is good, nothing odd there.

    My inclination is to just replace the rings, thinking that maybe the oil ring is just bad, but I cannot find any good reason why the oil ring should be that tight and odd-feeling.

    Pics/ Cylinder pic looks worse at top than it is, there is no scratching when you look by eye, the camera seems to have picked up something and exaggerated it.

    Anyone ever seen anything of the sort?





    Last edited by J Tiers; 04-17-2019, 10:50 AM.
    1601

    Keep eye on ball.
    Hashim Khan

  • #2
    Slip the oil ring into the cylinder and use the top of the piston to be sure it is sitting level, them measure the end gap to see if the wrong oil ring was used. If it is too tight the end can be ground to give proper clearance.

    Comment


    • #3
      Looks like a fair bit of scuffing in the bore and especially on the rings. I suspect it ingested some dirty air from a defective air filter at some point - doesn’t take much for this to happen. New rings and a light hone in the bore should work fine assuming the bore to piston clearances are within spec.

      Comment


      • #4
        Ring gaps when checked that way were normal and even on the high side, getting toward the rejection limit, but not there yet according to the Briggs repair books I have..

        Originally posted by Robg View Post
        Looks like a fair bit of scuffing in the bore and especially on the rings. I suspect it ingested some dirty air from a defective air filter at some point - doesn’t take much for this to happen. New rings and a light hone in the bore should work fine assuming the bore to piston clearances are within spec.
        It's not anything like the pic, actually. The camera with flash picked up things that cannot be seen or felt when looking at/feeling the bore. What looks like scratching at the top cannot be felt, and can barely be seen, the bore is as smooth as silk all throughout, seems to have acquired the usual finish that they get. The spot on the right side in pic is barely visible, and is not feelable either.

        And, as mentioned, it is only the oil ring that seems to be tight, but nothing shows up as out of the ordinary about it. Usually there is something that can be identified, but on this, there os nothing that seems out of the ordinary when looking at the thing.

        Piston is about 14 under nominal, at 2.236, but I do not have a spec for the piston size. seems just a little small relative to what I usually see, but that seems not to be an issue, it all focuses on the (perfectly good/normal seeming) oil ring. I was even wondering if this engine uses the backup spring on the oil ring, I think some do not, although I do not recall which if any. That spring sure tightens up the ring.......
        1601

        Keep eye on ball.
        Hashim Khan

        Comment


        • #5
          Two things - even if the pic makes things look allot worse im in agreement that the thing sucked at least a certain amount of dirt - there's allot of vertical lines,,,

          the other concern and something that might verify your statement that "it looks like someone might have been in it before"
          is the fact that it actually has horizontal lines, like someone gave it a hone WITHOUT knowing how to cross-hatch,,, you should never see horizontal lines there should only be 45 degree intercepting 45 degree's

          horizontal lines will "catch" and esp. on something like an oil ring that has thinner edges and high unit pressures...

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          • #6
            That's an interesting point. I can give it a light hone, for sure. And then decide if the rings are out of spec.
            1601

            Keep eye on ball.
            Hashim Khan

            Comment


            • #7
              And i would not reuse those rings - I would order up a new set - give it a "real hone" and assemble with some good break in oil and run that sow, dump the oil in short order and then go with some good quality lube done deal...

              Those rings are shredded on their running surface JT, and the cylinder bore does not look much better - they stick because they are like velcro when they settle the oil film out...

              Comment


              • #8
                +1 on what AK says. Someone didn't know how to hone a bore.
                “I know lots of people who are educated far beyond their intelligence”

                Lewis Grizzard

                Comment


                • #9
                  Originally posted by RMinMN View Post
                  Slip the oil ring into the cylinder and use the top of the piston to be sure it is sitting level, them measure the end gap to see if the wrong oil ring was used. If it is too tight the end can be ground to give proper clearance.
                  Ring end gap is an all or nothing deal, it's never "just a little too tight" for if the rings ends actually grow to a point that they start to connect just a little then things snowball real fast as it becomes a slight press-fit and then creates even more heat and growth factor,,, ring end gap either "lives or dies" in short order and you generally will never see it on already worn rings, it usually happens right from the get go when someone rebuilds an engine and does not check it...

                  if you could straighten a ring out and then measure it's length then it's easier to understand as to why so much gap/room is needed to begin with and also why the expansion factor goes bazzerk if they ever did touch,,,

                  One time I got called in to do "forensics" on a freshly rebuilt Ferrari that ended up seizing within the first 50 miles of it's life, under very close examination I found evidence that all top comprendo rings had "fret marks" where they actually did grow to the point where they touched together, when I seen that I knew it was the engine builders fault and my report stated so...

                  Edit; there is an exception to what I just wrote, ring end gap can be on the ragged edge and still never give a whimper of trouble for thousands of miles, like if someone was babying their engine for a long period of break in time and then after so many miles decided to go out and pull a colorado summit at full throttle high RPM's and created a ton of heat in the process, now you can seize up long after the rebuild ...
                  Last edited by A.K. Boomer; 04-17-2019, 12:44 PM.

                  Comment


                  • #10
                    Be sure to check the wrist pin that it’s free and tight. New rings for sure and by all means be sure to use the expander behind the oil ring and any other ring if supplied with the ring set. The expander helps apply extra pressure to the cylinder (but not enough to cause binding). The oil ring controls oil retention on the cylinder wall & scrapes excess oil from the cylinder which will bypass the other rings and wind up in the combustion chamber resulting in high oil consumption.
                    The rings should float freely on the piston with no binding. Carefully clean the piston grooves for a free fit. Do this until the rings are in fact free moving. If not then piston replacement should be considered.
                    Obviously check ring end gap. A rule of thumb is .004” per inch of bore.

                    Comment


                    • #11
                      RMinMN beat me to it. It sounds like ring gap on the oil ring is the culprit. Just for curiosity, I would give the oil ring more end gap and check for stiction, then throw them away and follow AK's ideas.

                      Sarge41

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                      • #12
                        If the oil rings end gap was actually so tight that it was dragging by hand turning of the crank it means the engine would not even be able to run at idle without complete seizure -
                        if end gap ever "connects" then you can consider it a press fit in short order, rings heat up along with pistons and combustion temps but if the ends ever touch then the rings heat goes supernova and snowballs and so does the expansion ratio, they either live - or seize there is no in-between...

                        Comment


                        • #13
                          Originally posted by J Tiers View Post
                          If will run, but not well.
                          Valves, carb, ignition. Nothing wrong with freshening up the rings & cylinder as long as you have it apart but not likely to be a big culprit.

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                          • #14
                            Wrong oil ring/oil ring expander for that particular piston?
                            And sorry, can't think of clever way of measuring it except the extreme case where ring groove depth is same or less as expander+oil ring thickness combined.

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                            • #15
                              You guys keep coming back to ring gap, but Jerry says more than once that the gaps were checked and all were normal or even edging toward too big. He says the oil ring is somewhat tight, but NOT because of too little end gap, more like too much spring. Did I get that right, Jerry?
                              Southwest Utah

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